Life as a Physicist

Energy vs Power vs Heat vs Oh no! July 5, 2009

Last post I mentioned the LHC update that was given at a recent meeting at CERN. One cool thing Steve Myers’ showed during his talk was a discussion of the quality of the splices and how it might affect the LHC’s ability to run.

For a sample of the trade-off, check out this plot, stolen from page 46 in the talk.

Along the x axis is the measured resistance between two magnets (across the splice). The units there are nano-ohms – something only the most expensive multimeters can measure. If you remember your Physics 101 course, you remember P=I^2R (power is current squared times resistance). The units of P are Watts (!) – just like your light bulb. These are superconducting magnets, of course. The magnets are very powerful and so have 1000’s of amps of current flowing through them. So even small R’s mean decent heat sources. Heat warms up the magnets, and makes them no longer superconducting – and that can be a disaster (a few of these is not a problem – it happens every now and then – but a chain reaction is what caused the last September accident). So – the splices, which aren’t superconducting, need to be excellent and have almost no resistance. Like 10-15 nano-Ohms.

The Y axis is how much current you are pumping through the magnet. Current is proportional to the magnetic field, which is proportional to the energy we can run the LHC at. As you can see, if you can run at about 6700 amps you can run at 4 TeV. If you run at 8300 amps then you can run at 5 TeV.

The red and green lines are the keys to reading this plot – they are two different conditions for the state of the copper joints. The LHC machine folks always talk about the worst case scenario (the red line) – but I’m not 100% what the difference between the two is. Lets say you want to run at 5 TeV. Follow the 5 TeV line over from the left of the plot until it hits the red line. You see that it drops down to 58 nOhms. That means all splices have to be less than 58 nOhms in order to run at this energy. The machine is full of these splices. So this is a bunch of work checking these guys!! [listen to the video on the agenda page at about 30 minutes in]. So, one of the things the LHC engineers are doing is measuring all the splice resistances and then putting them up on that plot to see where they are.

BTW, nominal is 10-12 nOhms, and they need to be less than 25 to run at a full 7 TeV (two beams at 7 TeV gives you 14 TeV, the design of the LHC).

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